1. Field of the Invention
The invention is directed to a hybrid hydraulic system for a work vehicle. The hydraulic system comprises an open center for providing hydraulic fluid to high flow functions, and a close center hydraulic circuit for providing hydraulic fluid to low flow functions.
2. Description of the Prior Art
Work vehicles, such as four wheel drive articulated loaders use hydraulic circuits to control or augment a number of functions, such as steering, loading, braking, controlling, etc. Deere & Company, the assignee of the present application, manufactures a series of four wheel drive articulated loaders. Deere loader models 444D, 544D and 644D are provided with two separate hydraulic circuits, each circuit is pressurized by fixed displacement engine driven gear pump. The first circuit provides hydraulic fluid to the braking functions whereas the second circuit provides hydraulic fluid to the steering and loading functions. The largest loader manufactured by Deere, model 844, has three engine driven fixed displacement vane pumps delivering hydraulic fluid to the steering and loading functions, and a separate engine driven gear pump providing fluid to the braking functions.
Fixed displacement pumps are used in open center hydraulic circuits. Fixed displacement pumps drive the same volume of fluid every cycle, and as such, fluid pressure varies with demand. Variable displacement pumps are used in close center hydraulic circuits. Variable displacement pumps maintain constant output pressure by varying fluid volume output of the pump. Typically fixed displacement pumps are less expensive than similarly sized variable displacement pumps. In addition, open center hydraulic circuits have a quicker response rate because of the constant flow of fluid.
Open center hydraulic systems are generally simpliest and least expensive to design. However, as more hydraulic functions are added, with varying demands on each function, the open center system requires the use of flow dividers to proportion oil flow between the functions. The use of flow dividers in an open center system reduces efficiency with resulting heat buildup.
Close center hydraulic systems with variable displacement pumps are better suited to more complex hydraulic systems because the quantity of oil delivered to each function can be controlled by line size, valve size or by orifice size with less heat buildup when compared to the flow dividers used in comparable open center systems. In addition, closed center systems do not need relief valves, thus, preventing heat buildup where relief pressure is frequently reached.
A number of attempts have been made to use a fixed displacement pump and a variable displacement pump in a hydraulic system to take advantage of each pump's best features. Fixed displacement pumps have been used as charge pumps for variable displacement pumps, see U.S. Pat. Nos. 3,659,419 and 3,785,157. In U.S. Pat. No. 3,659,419 the booster pump prevents cavitation at the suction side of the variable displacement pump while also providing a driving source for other elements. In U.S. Pat. No. 3,859,790 it has been proposed to use a variable displacement pump to drive a public works machine and a fixed displacement pump to drive the jacks actuating the working equipment of the machine. It has further been proposed, to combine the hydraulic output of both a fixed displacement pump and a variable displacement pump to pressurize the working circuits of a loader backhoe, see U.S. Pat. No. 3,962,870.